Optical imaging devices, like Single-Lens-Reflex (SLR) cameras, which are widely used in the field of photography, are usually equipped with an auto-focus system, by which the camera lens system automatically focuses the image on a selected part of the target subject. The detector for accomplishing the auto-focus (AF) function is usually located at the center of the frame because this position is generally more sensitive and more accurate than the other parts of the frame. Moreover, the center location for AF sensor is also preferred for usability reasons, and because it is more difficult for the peripheral areas to achieve the same optical performance as the center.
Under those circumstances, in which the part of the subject which should be focused, is not in coincidence with the center of the frame, a so-called “focus-recompose technique” is widely used. This technique involves the steps of firstly placing the central auto-focus detector on the portion of the subject that needs to be most clearly focused and activating, i.e. locking, the auto-focus in this first position of the camera.
Once the focus-lock has been activated, the camera is reoriented in a second step until the desired framing is achieved and finally the shutter of the camera is released in this second position.
The technical disadvantages of this known “focus and recompose” technique become apparent when the target subject goes out of the depth of field of the camera, e.g. in those applications which are directed to shallow depth of field situations, like portrait photography. The target object, on which the focus was locked in phase one, will shift away from the plane of focus as the camera is reoriented in phase two.
In order to circumvent the disadvantages associated with the focus-and-recompose-technique, it is already known from practical examples, e.g. from the single lens-reflex (SLR) camera type Canon EOS 20D, to use a variety of focus detectors instead of just one central focus detector to be able to skip the reorientation phase after focus lock. In these types of cameras it is advised in its technical descriptions to manually select one specific of the several focus detectors, e.g. that focus point which is already on top of the target subject.
However, the disadvantage of utilizing several focus detectors is that this procedure is typically not as fast and that it is not as easy to select the different focusing points compared to the use of a single central focus point.
Another disadvantage is that in many cameras the focus points outside the central focus point are not as accurate and sensitive, which leads to the result that in low-light conditions the focus cannot be locked by another than the central focus point. Thus, a larger focusing error results from using a less accurate auto-focus sensor.